Many applications require illumination of points on a target at specific illumination angles to achieve a desired contrast for examining the surface, for example to analyze the surface of electrical circuits on a printed circuit board (PCB). Typically in process of fabrication of electrical circuits, such as printed circuit boards, interconnect devices, Integrated Circuits and flat panel displays, an automated optical inspection operation is used to identify defects in the electrical circuits or on the substrate. Typically examination of a PCB is achieved by illuminating the surface of the PCB line by line with one or more line light sources and scanning the line for analysis. A line light source may be created by various methods, for example by forming a row of glass fibers that originate from a point light source (e.g. a halogen light), or by forming a row of LED sources. A lens may be placed along the line between the line light source and the PCB to focus the light in the plane perpendicular to the line to achieve an optimal angle of illumination on each point along the line.
Along the longitudinal axis each point of the line on the PCB is illuminated from the multiple light sources forming the line light source. The overall angle of illumination of the point is determined by the dispersion angles of the point light sources that the line light source is made up from, for example a Halogen light source tends to generate light that spreads out forming an angle of about +/−25° from the focal point of the source along the longitudinal axis. LED light sources generally generate light that spreads out forming an angle of about 70° in all directions.
To optimally analyze a point on the PCB it is desired to illuminate the point symmetrically, to eliminate the need to perform calculations to compensate for the asymmetry of the lighting. However controlling the illumination angles of the points that make up the line light source in all directions constitutes a geometrical challenge for lens makers. Such a solution may be impossible or very costly depending on the size and number of points forming the line light source. Therefore other methods of controlling the dispersion angles of the light are desirable.